Well...i must say that i almost got confused here!
i feel for you. you really have kinda case!
your exboyfriend is with your girlfriend.

i am sorry that i can`t offer much help as to how do you both not loose her but

"live with this all at the same time"

LIVE that`s what important! i wonder..doyou have any serious feelings for any one of them? whom do you love more?

maybe you don`t need such relationships right now.
i guess those guys both of your EXes- the boy and the girl-are alike in this point-they make take things easier than you do. they seem to be less responsible, less loyal. maybe. and thus maybe both of them aren`t a good company for you.
^
now i don`t make a statement here- please don`t take it so- i say that you may think of it and check it out. it`s a question....you may want to ask yourself.

let everythng you discover in this situation-both about them and about yourself-teach you. that`s what happenes to me right now, although not as real and as mixed up as with you. then, you come to understand things about life. next time you will learn to handle situations better.